Mice bearing methylcholanthrene-induced fibrosarcomas have lymphoid cells which suppress responses to cells of the same tumor in vitro. Thymus-derived (T) cells from the spleens of these mice synthesize a factor which blocks cytolysis of homologous tumor cells by specifically immune syngeneic lymphoid cells. I propose to continue to study the mechanism of this suppression and to determine if inactivation of these suppressor cells in vivo will augment rejection of transplanted tumors. Specifically, I will 1) determine whether the antigen-specific T cell suppressor factor directly suppresses cytolytic cells in a 51Cr release assay or whether it acts indirectly by inducing other suppressor cells; 2) define the phenotype of the subpopulation of T cells which synthesizes the suppressor factor and determine which cells are affected by it; 3) attempt to establish a stable line of hybridized cells synthesizing the suppressor factor; 4) attempt to raise an antiserum to the idiotype of the suppressor factor. My future goals are to determine if treatment with this antiserum and complement inactivated the spleen cells which synthesize the suppressor factor in vitro; and to determine if tumor-bearing mice treated with this antiserum have spleen cells which synthesize the suppressor factor and if growth of their tumors is retarded.